Pneumatic rubber passenger tires conventionally have a resilient, ground-contacting tread of a rubbery, elastomeric composition. The tread rubber is conventionally compounded to provide reasonable passenger comfort, good wear and traction.
However, for automobiles, as compared to larger cargo-carrying vehicles, increasing the energy efficiency of the tire by reducing its rolling resistance has become increasingly important. Such a reduction in rolling resistance for the tire in its inflated state under load would desirably reduce required engine fuel, thereby enhancing an energy savings, preferably without substantially or materially degrading the wear and traction features of the ground-contacting tread of the tire itself.
Tires with treads having portions of differing rubber compositions have been heretofore described for various purposes. For example, a tire with a tread of an outer cap and inner base of somewhat differing rubber compositions has been described (U.S. Pat. No. 3,157,218) as being useful for proving a tread of certain enhanced properties but with a tendency to resist excessive heat buildup. The resistance to excessive heat buildup is understood to enhance overall durability of the tire itself.
Pneumatic tires, particularly of the oversized or giant type for large vehicles as distinguished from passenger vehicle tires having treads with a central region and outer regions of differing rubber composition, have been described. (U.S. Pat. No. 3,364,965). Such tread is designed to have a central region with a synthetic rubber composition having a high coefficient of friction, apparently to enhance vehicular road-handling ability, and outer regions of natural rubber having a lower coefficient of friction.
It was also required that the central and outer regions be of substantially the same hardness. This regional trend design was described as being in contrast to large pneumatic tire treads which theretofore were all of natural rubber because of relatively high tire temperatures being normally attained.
However, a continuing interest remains for pneumatic tires, particularly pneumatic passenger tires, meaning tires for passenger vehicles as distinguished from oversized or giant tires for large vehicles, with relatively low rolling resistance to enhance a vehicular energy or fuel savings without substantially degrading the tread's wear and wet skid resistance.
In accordance with this invention, a pneumatic tire having a circumferential, sulfur cured rubber tread adapted to be ground-contacting, is comprised of an axially central tread portion occupying about 20 to about 60, preferably about 30 to about 55, percent of said tread and positioned between two tread portions of about equal size extending to the shoulder regions of said tire, axially positioned one on each side of said central tread portion, and correspondingly together occupying about 80 to about 40, preferably about 70 to about 45, percent of said tread, where
(A) the cured rubber compound of said central portion is composed of about 60 to 100 rubber hydrocarbon (RHC) butadiene/styrene rubber and, correspondingly, about 40 to about 0 RHC cis-1,4-polybutadiene rubber, characterized by having a hot rebound value in the range of about 55 to about 65 and a Shore A hardness in the range of about 58 to about 78 and further characterized, when a part of said tread on said pneumatic tire in its inflated state, by having tread wear, rolling resistance, and wet skid resistance values individually normalized to a value of 100; where
(B) the cured rubber of said outer tread portions is composed of about 80 to about 50 RHC natural and/or cis 1,4-polyisoprene rubber and correspondingly, about 20 to about 50 RHC 1,4-polybutadiene rubber, characterized by having a hot rebound value in the range of about 70 to about 85 and a Shore A hardness in the range of about 52 to about 65 and further characterized, when a part of said tread on said pneumatic tire in its inflated state, by having, as compared to the normalized values of said central tread portions, a tread wear value in the range of about 75 to about 110, rolling resistance value in the range of about 120 to about 140 and a wet skid resistance value in the range of about 80 to about 95; and where
(C) said overall tread composite on said pneumatic tire in its inflated state is characterized by having, as compared to the normalized values of said normalized central portion, a tread wear value in the range of about 90 to about 110, a rolling resistance value in the range of about 105 to about 135 and a wet skid resistance value in the range of about 90 to about 100.
Resulting Shore A hardness for the rubber composition for the tread's central portion is, on an incidental basis, typically in the range of about 58 to about 78 and in the range of about 52 to about 65 for the outer portions. It is considered that these hardness values are somewhat incidental and of a rather secondary nature to the ranges of rebound values for the respective tread portions for the purposes of this invention.
The hot rebound value (ASTM Test 1054-66) is a measure of a cured rubber's hysteresis or its tendency to generate heat or consume energy under load and working conditions. When the rubber is used as a tread, on an inflated pneumatic tire under load, the hot rebound value can also relate to rolling resistance of the tire itself, a reduction of which can effect an energy savings in the use of a tire on a vehicle such as a passenger automobile.
Thus, with somewhat comparable cured rubber compositions, one with a higher rebound value is indicated to have a lower hysteresis, a reduced heat generation or buildup, a reduced energy consumption and a reduced rolling resistance when used as a tread on an inflated tire under load.
However, such enhanced or reduced tire rolling resistance accomplished by using a tread rubber of higher rebound value can generally be somewhat detrimental to its treadwear and traction, or wet skid resistance.
Therefore, the pneumatic tire with the tread of sectionalized rubber composite positions as required by this invention is considered important for providing a reduced rolling resistance while also providing adequate tread wear and traction, or wet skid resistance.
The wet skid resistance is considered to be a more adequate measure of traction than dry skid resistance of an inflated tire's tread. The values were determined by building comparable tires with treads composed entirely of the rubber compositions to be tested.
The values determined for a tread of the composition of the central portion were simply normalized, or given a rating of 100.
Similarly, the values for treadwear and rolling resistance were also simply given a rating of 100 for comparison purposes to the other tread compositions.
The carcass to which the tread is adhered can be of a generally conventional construction of a generally toroidal shape with cord-reinforced carcass, two spaced, relatively inextensible, beads and sidewall portions extending from said beads axially outward to join the tread at a shoulder region. The rubber of the supporting carcass for the tread can be of various sulfur-cured types such as natural rubber, synthetic cis-1,4-polyisoprene, cis-1,4-polybutadiene, butadiene/styrene copolymers or mixtures thereof.
It is understood that the rubber composition for the tread can be compounded with various conventional compounding ingredients which include carbon black, processing oils, pigments, zinc oxide, stearic acid or a stearate, antioxidants, antiozonants, accelerators, retarders, resins, sulfur, or sulfur-providing compounds.